Missile guidance systems may employ a flexured mass accelerometer wherein a microwave cavity changes in length via movement of a proof mass when subject to acceleration. An oscillator radiates radio frequency (RF) energy in the cavity producing a standing wave known as a transverse electric or transverse magnetic resonance. The frequency of this resonance is a function of the length of the cavity. When subjected to acceleration, the proof mass moves changing the length of the cavity and also the frequency of the transverse electric or transverse magnetic resonance. A phase detector can then be used to detect the new frequency and a microprocessor is employed to calculate acceleration based on the frequency change of the transverse electric or transverse magnetic resonance.
One problem, however, is that the frequency of the resonance is also a function of the diameter of the cavity which can vary based on temperature.
In response, those skilled in the art have proposed employing temperature sensors to provide a correction factor or employing thermal control techniques. The use of temperature sensors alone is not often practical for very accurate systems because traditional temperature measurement devices such as thermistors are not sensitive or accurate enough in an operational environment to provide sufficient data for temperature compensation or control. Also, other error reduction techniques are often impractical due to the high material costs and the manufacturing limitations associated with low thermal expansion materials.
Those skilled in the art have also devised dual cavity systems wherein movement of the proof mass increases the length of one cavity and decreases the length of the other cavity. See U.S. Pat. No. 5,351,541 incorporated herein by this reference. An electromagnetic standing wave is formed in both cavities and when, the proof mass moves due to acceleration forces, one standing wave frequency increases while the other standing wave frequency decreases. Both frequency changes are indicative of both acceleration and temperature changes affecting the radius of both cavities. But, the effect of temperature on the cavity dimensions can be nullified using common mode error reduction techniques.
Such a dual cavity system is only accurate, however, if both cavities experience the same temperature effects. If both cavities respond equally, any temperature effects are eliminated during the frequency subtraction process. Unfortunately, due to slight variations in manufacturing and in the flexure stiffness of the proof mass, common mode error reduction does not completely remove thermal errors. In addition, certain errors, such as temperature gradients along the input axis, cannot be corrected for because the responses of the two cavities are intrinsically different.
Since one cavity may experience a temperature variation different than the temperature variation experienced by the other cavity, the system described in U.S. Pat. No. 5,623,098, also incorporated herein by this reference, discloses a single cavity design with two standing waves excited in the cavity. One oscillator produces a standing electromagnetic wave in the cavity (as in the prior art) which changes in frequency as a function of cavity length and as a function of cavity diameter. The other oscillator produces a specific transverse magnetic resonance in the same cavity which changes in frequency only as a function of cavity diameter. The frequency at which a transverse magnetic resonance is produced is known to change with cross-sectional area changes of the cavity, but for a certain subset of these resonances, not with cavity length. Thus, frequency changes of the specific transverse magnetic resonance in the cavity can be used to compensate for temperature induced errors.
The problem with this design, however, is that it erroneously assumes there are no temperature gradients in the single cavity. And, although the '098 patent teaches away from a dual cavity design, single cavity design will not work well in high accuracy applications because of the stringent requirements placed on the system clock that must be radiation hard. With two cavities, when the resonant frequencies are chosen appropriately, the system clock must be stable to one ppm while with only one cavity the clock must be stable to the order of one ppb.